1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which performs both image display mode and fingerprint recognition mode. The liquid crystal display device according to the present invention is more effective than a piezoelectric sensor or an optical sensor which uses one or more semiconductor components and is generally employed in fingerprint recognition-based security/authentication techniques, in view of reliability and costs. Furthermore, the present invention is directly applicable to STN or TFT-LCD products which themselves have a very wide applicability, so that it can be employed in an electronic commerce system, a security system, a personal recognition/authentication system or the like.
2. Description of the Prior Art
As generally known in the art, password input mode has been principally used for electronic commerce, security, authentication and the like, up to the present. However, since the password input mode is likely to be hacked into, various modes have been developed lately for recognizing biometrical information such as fingerprints.
FIG. 1 is a longitudinal section view of a unit cell in a conventional fingerprint recognition device which employs thin film transistors (TFTs), and FIG. 2 shows a circuit for driving such a fingerprint recognition device. As shown in FIG. 1, a unit cell of the conventional fingerprint recognition device 100 comprises: a sensor TFT 102 for sensing light; a switch TFT 106 for outputting recognized fingerprint information, the sensor TFT and the switch TFT being transversely aligned; a transparent substrate 118; and a backlight 116 for emitting light upward from the underside of the transparent substrate 118, the light passing through an electricity charging unit or a light transmission part 104. A sensor source electrode 112 of the sensor TFT 102 and a switch drain electrode 128 of the switch TFT 106 are electrically connected to each other through a first transparent electrode 124. A second transparent electrode 120 is connected to a sensor gate electrode 114 of the sensor TFT 102. In addition, a photosensitive layer 110 such as amorphous silicon (a-Si:H) is formed between the sensor drain electrode 108 and the sensor source electrode 112, so that the sensor drain electrode 108 and the sensor source electrode 112 become electrically conductive if a predetermined amount of light is incident into the photosensitive layer 110. If a fingerprint is in contact with a coating 126 formed on the top of the unit cell, the light generated from the backlight 116 underneath the transparent substrate 118 is reflected along the pattern of the fingerprint and received by the photosensitive layer 110 of the sensor TFT 102, thereby rendering the TFT 102 to be electrically conductive. A dielectric insulation film 126 functions to isolate the second electrode 124, the sensor gate electrode 114 and the switch gate electrode 136.
Meanwhile, the switch TFT 106 is switched frame by frame, the frames being set to scan a fingerprint by a gate control signal applied to the switch gate electrode 136. Consequently, each sensor TFT 102 scans a fingerprint image inputted into the fingerprint recognition device 100, thereby forming a frame. The fingerprint image scanned in this manner is outputted via the switch source electrode 132. A photosensitive layer 134 is also formed in the switch TFT 106 as in the sensor TFT during the manufacturing process of the fingerprint recognition sensor, but a light shut-off layer 129 is formed on a protective layer 130 so that the switch TFT 106 is not turned on by the light received into the photosensitive layer 134.
Referring to FIG. 2, if a TFT sensor 202, which consists of a light-emitting unit 204, a panel 206 and a coating 208, is turned on by a gate drive unit 210 to scan a fingerprint as described above, fingerprint image information is inputted into a reading unit 212, sent to a control unit 214 and then compared with fingerprint data which has already been inputted into a memory 216. The result of the comparison is sent to a sensor interface 220 of a host computer, so that a process related to security and authentication then proceeds.
FIG. 3 is an equivalent circuit diagram for an array of conventional fingerprint recognition components. As shown in FIG. 3, a unit cell comprises a sensor TFT 302 and a switch TFT 304, and the capacitance existing at the connection between the sensor TFT 302 and the switch TFT 304 is modeled by a capacitor 305. As shown in FIG. 3, lines 306_1 and 306_2 are connected to the gate of the switch TFT 304, and line 308 is connected to the gate of the sensor TFT 302. Line 310 is a data line of the sensor TFT 302 and line 312 functions to outwardly discharge static electricity which may be generated in the light shut-off layer 129 (FIG. 1).
The afore-mentioned fingerprint recognition devices should be separately provided in an electronic commerce system, a security system, a control system and the like. Recently, in connection with the increase of personal portable equipment, mobile phones, personal portable terminals, notebook computers, personal computers and the like, various application techniques have been developed for connecting a fingerprint recognition device with such equipment. However, there is a problem in that the price and volume of a resulting product are increased because it is necessary to buy and mount a separate fingerprint device on a liquid crystal display panel or in a separate space.